1. Field of the Invention
This invention relates to an image forming method which expresses so called image informations in two dimensions such as letters, figures, graphs, images, etc. in a visually sensible form, and particularly to an image forming method which expresses image informations having gradation characteristics.
2. Description of the Prior Art
As methods of expressing an image information having a gradation characteristic, namely an image information containing half-tone information in a visually sensible state, there may be mentioned as an example of the prior art a method in the field of, for example, image information recording, in which image information is expressed by controlling the number of image forming elements having a certain optical density and a certain area to be provided within a desired certain minute area such unit area hereinafter called a cell) constituting an image element and the state of arrangement of said image forming elements within said image element.
That is, according to said example of the prior art, for example, one image element may be sub-divided into a matrix of plural number of a cells which can be each occupied by only one image forming element (hereinafter called as cell matrix), and a desired level of gradation characteristic can be expressed depending on the number of cells occupied by the image forming elements and the arrangement of the image forming elements occupying the cells among the cells constituting said cell matrix. And, the image forming elements are arranged necessarily with arrangement pitches which are as large as the minimum arrangement pitch determined by the size of the cell (hereinafter called the standard arrangement pitch Ps) multiplied by a whole number (this is hereinafter expressed as the image forming elements being arranged under a quantized arrangement pitch).
According to this method, in order to enhance the gradation characteristic, the area of one image element per se (image delivery area) is required to be enlarged, or the area of the image forming element is required to be made as small as possible to increase the number of cells constituting one image element.
However, in the former case, it is an undeniable fact that sharpness or resolution is lowered to a greater extent as the area of the image element is increased, which makes the image coarse, and therefore this approach is not desirable from a practical point of view. Also, in the latter case, when the cell number constituting one image element is attempted to be increased while maintaining extent of sharpness or resolution, the extent of improvement is inherently limited due to the limitation with respect to reduction of the area of the image forming element.
Accordingly, when it is desired to express a gradation in a broad range from a gradation level of higher optical density to a gradation level of lower optical density, the gradation expression obtained lacks in fineness of texture due to a deficiency in gradation level numbers only giving images which provide a feeling of "graininess" or "roughness". On the other hand, when the gradation characteristic is attempted to be expressed with fine texture, either the highlighted portion or the dark portion cannot be expressed due to the breadth of the gradation expression which cannot be made so broad, whereby the resultant image is such that a feeling of "glittering" is imparted to the highlighted portion to give a stimulating image, when the gradation characteristic at the dark portion is intended to be expressed sufficiently well, while the image as a whole becomes lower in tone and lacking in sharpness, when the gradation characteristic at the highlighted portion is intended to be expressed sufficiently well. In short, in either case, no attempt to express the gradation characteristic with fine texture will be successful in well-balanced selection of the region to be expressed, thus giving only an unnatural image.
In particular, in case of photographic images, unnaturalness is marked in reproduction of the highlighted portion or the skin portion of a person, and it is difficult to express half-tones smoothly and continuously over a wide range.
On the other hand, for example, in an ink jet recording method wherein image recording is performed by forming flying ink droplets and permitting said ink droplets to be attached on a material to be recorded (e.g. sheet material such as of paper, plastics, ceramics, etc.), thereby forming ink dots (one of the image forming elements), it has already been proposed to use liquid or solid inks with different color densities (for each color in case of color recording) which are to be employed in different manners to carry out recording.
And, when image portions with equal optical reflective densities at a certain area (hereinafter written as "average reflective density") are to be expressed, two expressions may be possible, one being the expression by forming dots with small area with a high density (high color density) ink under a desired dot pitch (arrangement pitch of dots), and the other being the expression by forming dots with large area with a low density (low color density) ink (of course, use of inks with different densities is not limited to the two kinds, but two or more kinds of inks, and two or more kinds of colors may also similarly be referred to, although this explanation is only of the case in which two kinds of inks of high density and low density are employed). However, even if the average reflective densities of the two images expressed may be approximately equal to each other, viewing of the finished image will give an impression to the human eyes as if there were a great difference in the quality of the image. Of course, such a feeling will vary depending on the color density and the dot pitch, but, generally speaking, a "grainy" feeling is strengthened in an image expressed with formation of small area dots by use of a high density ink, which has been frequently a great cause of a loss naturalness of the image in general image expressions.
On the other hand, when the expression of the minimum optical density (the lowest gradation level) in the image expression is intended to be expressed without formation of dots, there is formed a white drop-out portion on the image, which white drop-out portion is clearly changed in tone from the image portions in which other dots have been formed, thus being a factor in lowering image quality.